This invention relates generally to the correction of aberration in charged particle beams and, more particularly, to the correction of chromatic aberration in charged particle beams. The term chromatic aberration is used in particle beam physics to describe phenomena relating to beam particles of different energies. When charged particle beams are focused or deflected using magnetic or electric fields, the effect of a focusing or deflection field on the particles is dependent on the energy of the individual particles.
In many applications of particle beams, there is a requirement for beam steering over a wide range of angles, as well as for focusing the beam. Because of the energy spread of the beam, deflection through any significant angle causes beam dispersion. This distortion of the intended beam divergence is referred to as chromatic aberration. It is analogous to chromatic aberration in optical systems, which is caused when light of different wavelengths is refracted through different angles because the transmission medium, typically a lens, exhibits different indices of refraction at different wavelengths. Chromatic aberration in lenses can be corrected to some degree by forming composite lens systems of different materials. This approach has no direct analogy in particle beam physics. In the past, the correction of chromatic aberration in particle beams has relied on fairly complex arrangements of electric and magnetic fields, but no practical solution to the problem has been found, especially for high-energy beams.
It will be appreciated from the foregoing that there is still a significant need for a technique for correcting chromatic aberration in charged particle deflection systems. The present invention satisfies this need.